Wavelength division multiplexing (WDM) systems typically require tunable optical filters, such as Fabry-Perot filters, switches, interference filters, and variable optical attenuators, along with other devices that employ highly reflective mirrors.
Forming thin film highly reflective and/or low absorption mirrors involves the application of multi-layer thin dielectric film coatings on a substrate. The refractive index differences associated with certain material systems such as ceramics require the formation of 10 or more layers to achieve a reflectivity greater than 97%. When the substrate is an optical membrane, such as found in micro-optical electromechanical systems (MOEMS), the coatings can be one-half or more of the thickness of the substrate.
It can be important to maintain a flat or selectively curved reflective surface and to prevent the application of the thin film from forming any xe2x80x9cbowxe2x80x9d or a controlled bow in the substrate. However, the thinness of some of the membranes makes them highly flexible or susceptible to bow, such that even small surface stresses can lead to deformation, which impacts the optical properties of the resulting device.
The application of highly reflective thin film coatings especially to thin flexible substrates can lead to difficulties arising out of the inherent residual stresses that are generated when the thin films are formed, such as by thermal evaporation or sputtering. For example, the columnar microstructures created during conventional thin film formation can lead to electrostatic attraction between adjacent xe2x80x9ccolumnsxe2x80x9d. This can result in an undesirable tensile stress being present in the thin film. Such stress can affect the flatness of the membrane and thus the optical properties of the resulting device.
Since each layer develops its own inherent stress properties, forming dielectric coating layers without bowing the thin substrate has proven to be a difficult goal to achieve.
Generally speaking, in accordance with the invention, a membrane, or release structure, having a thin film optical coating thereon which is formed from multiple layers of different materials is provided in which the net stress of the thin film coating on the membrane is low, such as about 10 MegaPascals (MPa) or less. Such films are formed through thermal evaporation, with ion assist, by directing an electron beam at a source, in a known manner, to raise the coating material temperature and evaporate material from the source onto a thin flexible membrane with the assistance of a coincident ion stream. The ion beam current is proportional to the arrival rate of ions and should be made sufficient to provide a thin film coating that has substantially no porosity.
Successive applications of layers of material are preferably deposited at different ion gun voltages and/or current. One embodiment describes a method of utilizing a static ion gun voltage and current to produce a multi-layer coating, which has a predetermined stress value. This is made possible by experimental determination of a stress map, in which the stress of the thin film coatings or layers of thin films deposited at voltages is evaluated and mapped and the voltage at which the stress is substantially 0, or not more than about 10 MPa identified. This voltage, corresponding to the desired stress characteristics, is used to provide additional controlled stress coatings.
The above and other features of the invention including various novel details of construction and combinations of parts, and other advantages, will now be more particularly described with reference to the accompanying drawings and pointed out in the claims. It will be understood that the particular method and device embodying the invention are shown by way of illustration and not as a limitation of the invention. The principles and features of this invention may be employed in various and numerous embodiments without departing from the scope of the invention.